The invention relates to a magnetic sensor for the absolute counting of revolutions or linear distances, which advantageously can be used in multifaceted fields of technology, and in particular in automotive engineering and in industrial automation.
Sensors for determining an angular position according to various physical principles are used widely. They all have in common that the sensor signal is periodic after 360°, which is to say the sensor cannot distinguish between 10° and 370°. Such sensors are therefore needed for tasks in which the angle must be determined beyond 360°, as is the case with the steering wheel in the automobile, for example, combined with a further sensor, which must be able to detect the number of revolutions. In combination with a revolution counter, it is then possible to distinguish between 10° and 370°. So as to determine the number of revolutions, solutions are known in which the number of revolutions (such as between 1 and 5) can be inferred mechanically via the turn of a spiral having N spiral arms. Other solutions utilize mechanical gears in conjunction with two or more angle sensors. Having knowledge of the construction of the gear and the angular positions of the magnets connected to different wheels of the gear, it is also possible to determine the angle from 0 to 5·360°, for example. All these solutions have in common that they require a mechanism for implementation and as a result they are not non-contact, and thus not wear-free. However, a non-contact solution is necessary for many applications, in particular in automobiles. This could be implemented by determining the angular position at every point in time (permanently) and in this way being able to distinguish a transition from 359° to 360° from an angle of 0°. This requires that the sensor and an associated memory element are permanently supplied with electric energy. This is inconsistent with the requirement in automotive engineering that the determination of the absolute angle in the range of 0° to 5·360°, for example, must also be successful when the onboard electronic system is disconnected from the battery, for example.
The company Posital developed non-contact counting of the number of revolutions that satisfies these requirements in principle (company announcement “Kraftwerk im Encoder . . . ” (Power house in the encoder . . . ) www.posital.de). A Hall sensor is used there for determining the angle (0 to 360°). The number of revolutions are measured using what is known as a Wiegand wire. This wire has special magnetic properties that ensure that after every revolution, due to the discharging sudden movement of a magnetic domain wall through a wire that is a few millimeters long, a brief but sufficiently intense voltage pulse is created, which can be written to a ferroelectric random access memory (FeRAM), even without the FeRAM being connected to the battery. This solution thus satisfies the demand for the wear-free and non-contact determination of the number of revolutions and also counts revolutions up to the maximum memory capacity of the FeRAM without the current supply being applied. However, the automobile industry rejects this type of solution because cost-effective production and assembly are not possible given the macroscopic size of the Wiegand wire, and problems exist with electromagnetic compatibility due to the high-resistance input of the FeRAMs.
A further sensor element for counting revolutions that satisfies the above-mentioned requirements is known from EP 1 740 909 B1 (WO 2005/106395). This sensor element has the shape of an elongated spiral having N windings and is composed of a stack of layers that has the giant magnetoresistance (GMR) effect. The GMR layer system of this sensor element is substantially composed of a hard magnetic layer, which defines the reference direction, and a soft magnetic layer, these being separated by a non-magnetic intermediate layer. The outer rotating magnetic field to be detected is strong enough to change the magnetization direction of the soft magnetic layer due to the movement of the domain walls, but it is too weak to change the magnetization direction of the hard magnetic layer, which runs parallel to the straight sections of the elongated spiral. The sensor element thus responds to a rotating magnetic field with a change in resistance, wherein whole and half revolutions are detected in the form of 2N+1 resistance values within the countable range of 0 to N revolutions. Each resistance value is thus bijectively assigned to a half-integral or integral revolution value. The magnetic structure remains unchanged if the magnetic field does not rotate. In the case of a rotation, the magnetization directions change, regardless of whether the resistance value is read out or not. This means that the system detects all changes of the rotating magnetic field even in a current-less or power-less state, and current supply is only needed for read-out, which is to say for determining the resistance.
The disadvantage of such an arrangement is that, due to the memory geometry used (each revolution requires a complete spiral winding), the spiral must be very large geometrically when counting a larger number of revolutions. As a result, the probability increases that defects that occur during production of the spiral will lead to failure, and thus to a reduction in the yield. In addition, the chip surface area increases, and along with it the costs for such a sensor. Moreover, when the number of spiral windings is large, the concept provided in EP 1 740 909 B1 automatically results in problems in determining the number of revolutions. The usable voltage swing, which results from one revolution to the next, is scaled at 1/number of spiral windings. This swing is clearly too small for a reliable evaluation for N> to >>10. One alternative, which is provided in the aforementioned patent, does permit the full magneto-resistance swing at higher numbers of revolution, but likewise has the disadvantage of a long spiral, and the advantage of the large swing comes at the expense that, instead of two electrical contacts, all spiral parts that form a non-closed circuit must each be provided with four electrical contacts, and be read out and processed electrically. Even at N=100, this is four hundred contacts, and thus the circuitry is very complex. The above-described solution entails the added problem that, once the maximum number of domains that can be guided in the spiral conductor is reached, the conductor is entirely populated by domains, and during any further semi-rotation one domain leaves the conductor, while a new one is fed at the same time. This ends the unambiguous nature of the revolution counting at n windings, and thus 2n domains. A directionally reversed revolution of the outer rotating magnetic field ultimately clears the spiral completely of domains, so that unambiguous counting also ceases to be possible in the reverse direction once the maximum detectable revolutions are exceeded.
Eliminating the above-described problems was already the task of one proposal according to WO 2009/027046 A1, in which a magnetic revolution counter for the unambiguous determination of a predefinable number of revolutions to be determined for a rotating element was proposed, in which, depending on the number of revolutions to be measured for the element to be detected that is provided with a magnetic system, the magnetic field of which permits the detection of all provided sensor elements, a plurality of sensor elements are provided, wherein the sensor elements were populated by magnetic domains having a predeterminable and fixed number. According to this proposal, the domains are guided in respective closed loops, which include at least one ferromagnetic and one soft magnetic layer, wherein tapered protuberances oriented into the interior of the loop were assigned to the loops, and the number of protuberances provided per loop is established in a defined manner deviating from each other from loop to loop. With the aid of electrical contact arrangements provided there, which allow the changes in the electrical resistance of predefinable loop sections to be detected after magnetic domains have changed location as a result of the action of the outer rotating magnetic field of the magnet system in the predefined loop sections, it is possible to supply these resistance values to an evaluation unit for the purpose of correlating the number of revolutions of the rotating element. The respective closed loops provided in this proposal can be nested in one another or be disposed adjacent to one another on a substrate. This solution solved the above-described problems of the necessary voltage swing and of the bijectivity of the counting within the predefinable boundaries, while reducing the overall conductor length at the same time. The tapered protuberances required within the scope of this proposal, however, represent considerable requirements with regard to the production technology. The reason is that the cusps must be implemented very precisely and at an angle of less than 15°. If one does not intend to use technologies such as focused ion beam (FIB), which are very complex and consequently also very expensive, the limits on achievable yield will be reached very quickly, at least with larger cusp numbers using standard lithographic technologies. This proposal is therefore only conditionally suited for large inexpensive sensor batches with little waste.
Moreover, another problem exists with the present solution, which is a domain conductor width that is not consistent throughout. In this regard, first the following fact must be pointed out, which applies to all sensors of this type: An upwardly and downwardly limited magnetic field range exists for the magnetic field acting on the sensor, in which reliable functionality of the sensor or system is achieved. Only above a minimum magnetic field (hereafter Hmin) is a domain moved 100% through the structure, and an encounter of two domains reliably prevented, along with the attendant destruction of the same, and thus the undesirable reduction in the number of domains. At the same time, however, the magnetic field must not be so large that magnetic domains are unintentionally created. This means that a magnetic field Hmax exists, which must not be exceeded. The field range of the sensor must therefore always be above Hmin and below Hmax. It is advantageous for any intended use of the sensor if Hmin is very small and Hmax is also very large, and thus the so-called magnetic window ΔH=Hmin−Hmax is as large as possible. The magnitude of Hmax is primarily dependent on the cross-section of the portion of the used layer stack in which the magnetic domains move. It is proportional to the thickness thereof, and indirectly proportional to the width thereof. The minimum field depends on the roughness of the layer. At a constant absolute roughness, Hmin decreases as the width increases.
It is now apparent in the proposal according to WO 2009/027046 A1 that, at the point of convergence at the lower end of a cusp, widening to at least double the domain conductor width is inevitable, resulting in a significant reduction of the upper field Hmax. When further assuming a typical minimally producible radius of curvature at this end of the cusp of 200 nm (which is a typically achievable value for a standard DUV wafer stepper), the width increases to ˜600 nm. This widening results in a massive reduction of the upper maximally permissible field of the magnetic window in which the sensor operates. In a first approximation, this value is indirectly proportional to the strip width (here =200 nm), and is thus reduced to a value of ˜33% of the value that would in fact be theoretically possible with the strip width of 200 nm.
The latter problem of the effect of varying conductor widths was solved in DE 10 2010 022 611 A1 by using a soft magnetic loop structure that is populated with a predeterminable number of magnetic domains and provided with GMR or TMR layer assemblies, wherein the loop structure is formed of at least two separate loops, which are each spiral-shaped, wherein the respective first inner loop end is connected to the respective second outer loop end of the same loop so as to bridge all the remaining loop sections of the respective loop at a predefinable distance by way of a respective soft magnetic bridge, which thus magnetically closes the respective loop, wherein at least one domain is written into each of the closed loop structures. This solution, which comes closest to the present invention, has a larger magnetic window than the comparable solutions known until then, since it at least allows conductor widths that are consistent throughout to be produced. However, creating the bridges provided there, and thus ensuring uniform thicknesses of the conductors in the step-like transition regions, poses such considerable technological demands that a mass production of such revolution counters is associated with an excessively high scrap rate, and therefore is uneconomical.
In addition to the above-described prior art, reference shall be made to document DE 10 2008 063 226 A1, which describes where and how an effective attachment of read-out contacts is to be carried out using the example of a rhombic spiral structure of domain-guiding conductors. Such deliberations are also used in the invention newly proposed herein, so that this would have to be addressed in greater detail only conditionally here. DE 10 2010 022 611 A1 likewise provides suggestions for an advantageous contact connection for reading out the sensor and for the defined writing of domains into the conductor structures, which can likewise be used analogously in the present invention.